1. Field of the Invention
The present invention relates generally to rotary drag bits used in drilling earth formations during exploration for and production of oil and natural gas; and, particularly, to such bits having cutting elements with synthetic diamond cutting surfaces.
2. Description of the Prior Art
Conventional rotary drag bits usually comprise a bit body having an upper end adapted to be attached to the lower end of a drill string. The lower end of the body defines the head portion of the bit which includes a plurality of cutting elements mounted thereon and projecting outwardly from the body for contacting and drilling through the earth formations. The cutting elements may consist of teeth made of tungsten carbide, or they may consist of a layer of natural or synthetic diamonds bonded to a slug, preferably made of tungsten carbide. Generally, such slugs are substantially cylindrical with one end having a planar surface for mounting the diamond cutting surface. The cylindrical portion is adapted to be pressed into bores formed in the head portion of the bit body and positioned to have the cutting surfaces facing in the direction of rotation of the bit. Also, the synthetic diamond cutting surfaces may be cast in place during the formation of the head portion or brazed in place on the head portion. As the bit body is rotated, the diamond cutting edges remove the earth formation at the borehole bottom.
As the diamond cutting surfaces must extend outwardly beyond the body, they are readily exposed to contact. And, as the diamond surfaces are rather brittle, due to their extreme hardness, the cutting surfaces are frequently chipped or broken when the bit is not handled with care. The cutting surfaces can also be easily damaged when the bit is dropped into a bit breaker, which is used to tighten the threaded connection when the bit is attached to the drill string.
Also, the synthetic diamond cutting surfaces can be easily damaged by chipping or breakage when the bit is inadvertently allowed to "tag" bottom (i.e., when the bit is rammed into the bottom of the borehole or as it nears bottom, if the drilling string is rapidly stopped, the drill pipe can stretch, allowing the bit to impact the hole bottom). The damage to the diamond edges can result in the complete loss of effectiveness of the cutting surfaces.
Prior techniques for protecting the cutting surfaces on rolling cutter bits and conventional diamond drag bits (i.e., bits having surface set natural diamond stones) have primarily utilized a bit protector made of a plastic, epoxy, or acrylic material which was molded onto and completely covered the rolling cone cutters or face of the diamond bit and shaped in such a fashion as to permit the easy passage of the bit through the borehole. Other prior techniques for protecting the cutting surfaces have utilized bit protectors made of wood chips or plastic that were molded to fit the contour of the rolling cutters or the diamond drag bit and held in place on the bit by straps or wire ties. Examples of prior art protectors can be found in U.S. Pat. Nos. 2,296,939; 2,644,672; and 3,788,407.
Disadvantages of these prior techniques for protecting the cutting surfaces are the difficulty of obtaining unobstructed circulation paths with the molded-on types and inadequate assurance of removal of the protector once the bit reached bottom. Disadvantages of the strap-on type protectors are the additional metal wires or straps (i.e., junk) in the hole which could damage the bit. Further, such chunks can also plug part of the annular circulation return past the bit.